1. Field of the Invention
The present invention relates to a DC motor driving apparatus, and more particularly, to a DC motor driving apparatus which drives a DC motor such as a three-phase brushless motor and performs the function of compensating for a change in drive voltage.
2. Description of the Related Art
Generally, with regard to a DC motor, given that the voltage applied thereto is constant, its rotational speed decreases with an increase in the load torque, and with the load torque being constant, the rotational speed is substantially proportional to the applied voltage. A DC motor having such a characteristic is used as, for example, a toner motor in a developer for use in a copying machine. A DC motor driving apparatus having the following structure is known as a driving apparatus for such a DC motor which can maintain the rotational speed of the motor even when the source voltage or load torque is changed.
The above type of DC motor is supplied with a drive current from a DC power source, and the supply of this drive current is controlled by a switching circuit. The switching operation of the switching circuit is controlled by an ON/OFF signal produced from a signal generator, thus controlling the current supplied to the DC motor. The rotational speed of the DC motor is detected by a rotational speed detector that uses a frequency generator, etc., and a frequency signal proportional to the rotational speed of the DC motor is acquired from the detector and is converted into a voltage value before being supplied to a speed comparator. The speed comparator compares the output signal (voltage value) of the speed detector with a predetermined speed reference signal (voltage value) and supplies a control signal corresponding to the difference between these two signals to the signal generator. In accordance with this control signal, the signal generator changes the duty (ON/OFF ratio) of the ON/OFF signal that should be supplied to the switching circuit. This controls the ON/OFF time of the switching circuit, thereby controlling the rotational speed of the DC motor.
With regard to a DC motor driving apparatus that performs a phase synchronizing control, a phase detector, phase comparator and adder are added in parallel to the aforementioned speed detector, so that both the rotational speed and phase of the DC motor are controlled.
More specifically, a frequency signal proportional to the rotational speed of the DC motor is acquired by means of the phase detector using a frequency generator, etc. and it is then supplied to the phase comparator. The phase comparator compares the phase of the output signal of the phase detector with that of a predetermined reference signal (reference frequency signal) and supplies a control signal corresponding to the phase difference of these two signals to the adder. The adder then adds the control signal from the speed comparator and the control signal from the phase comparator and supplies the resultant signal as a control signal to the signal generator.
With a thus constituted conventional DC motor driving apparatus, however, the control signal generated from the signal generator to control the rotational speed of the DC motor does not vary unless the rotational speed or phase of the motor actually changes due to a change in voltage from the power source or load of the motor. Further, in a case where a non-stable power source of lower cost than that of a stable power source is used as the mentioned power source or where the source voltage is changed with another load than the motor being coupled to the same power source that drives the motor, since the control signal from the signal generator determines only the ON/OFF duty, it is influenced by the non-stable power source or the mentioned load other than the motor. In other words, with the use of the non-stable power source as the actual power source, the voltage after the ON/OFF control is executed using the control signal may be varied or the voltage after the ON/OFF control may be varied due to a change in current which is caused by the influence of the load other than the motor. As a result, the rotational speed of the motor cannot be properly be controlled and may easily be changed.